Remove Node::documentInternal() and inline it into Node::document()

There remain no documentInternal() callers that could deal with a NULL
Document*, so those callers can simply use document() instead.

R=esprehn@chromium.org
BUG=331922

Review URL: https://codereview.chromium.org/123743008

git-svn-id: svn://svn.chromium.org/blink/trunk@164537 bbb929c8-8fbe-4397-9dbb-9b2b20218538